Acoustic measurement systems are used in a wide variety of applications, including industrial applications such as in manufacturing, motion detection for security systems, and increasingly in automotive applications, among others. In automotive applications acoustic measurement can be used for detecting the distance, as well as the rate of change of distance, between the vehicle and objects in the vicinity of the vehicle, such as other vehicles. This is particularly useful in collision warning/avoidance applications and parking assist applications. Acoustic pulses are transmitted from the vehicle and any obstacles within sufficient range reflect the acoustic pulses. The echo time and Doppler frequency shift of the reflected pulse can be used to derive information about the distance to the object as well as whether the object is moving towards or away from the vehicle, and at what rate.
Unlike relatively controlled environments such as manufacturing environments, automobiles operate in a wide variety of operating conditions. A given vehicle may be expected to operate in temperatures from extremes of −40 centigrade to +85 degrees centigrade, or more. Furthermore, weather conditions can be expected to include rain, dust, ice/snow, and so on. All of these operating conditions can affect the operation of an acoustic measurement system. In particular, the acoustic transducers used in such systems are typically an ultrasonic piezo transducer. The transducers have a natural or resonant frequency which typically varies over temperature. Furthermore, mechanical loading due to debris (ice, dust, etc.), and aging, wear, and damage of the transducer element can likewise affect the resonant frequency. As a result, the resonant frequency of the transducer can shift significantly even over relatively short periods of time in automotive applications, such as during the course of a routine commute.
The shifting resonant frequency of the transducer presents a problem. When conducting distance measurement operations, the transducer is typically driven at a frequency close to the resonant frequency. If the measurement frequency is too far (in frequency) from the resonant frequency, the frequency response of the transducer can attenuate the measurement signal to a level that is not practical.
Setting the measurement frequency to a fixed frequency where the resonant frequency varies, such as in automotive applications, can result in the resonant frequency changing to be too far away such that the frequency response changes to an unacceptable level. A conventional solution to temperature drift is to add temperature compensation to the driving oscillator to substantially match the expected temperature drift of the resonant frequency of the transducer. However temperature compensation does not remedy other causes of frequency drift, such as mechanical loading of the transducer, wear and aging, and damage. Temperature compensation can be difficult to achieve in applications such as automotive applications where temperatures can vary significantly in different areas of the vehicle. Accordingly, there is a need for means by which the operating frequency used for performing distance measurements in an acoustic measurement system is adjusted with the resonant frequency of the acoustic transducer as the resonant frequency varies with operating conditions.
A self-tuning acoustic measurement system adjusts it measurement frequency by driving an ultrasonic transducer of the measurement system with a driving signal from a driving circuit, ceasing the driving signal, thereby inducing the ultrasonic transducer to resonate at a present resonant frequency of the ultrasonic transducer, determining the present resonant frequency of the ultrasonic transducer by a receiver of the ultrasonic distance measurement system, tuning the driving circuit to a measurement frequency that is within a predefined bandwidth of the present resonant frequency, and conducting the distance measurement operation by driving the ultrasonic transducer with the driving circuit at the measurement frequency.
The self-tuning acoustic measurement system can include an acoustic transducer having a resonant frequency, a driving circuit coupled to the acoustic transducer which provides a driving signal to the acoustic transducer, a frequency determination circuit coupled to the acoustic transducer which determines the resonant frequency from a resonant signal produced by the acoustic transducer upon the driving circuit stopping the driving signal, and further indicates the resonant frequency to the driving circuit. The driving circuit sets a measurement frequency used to conduct measurement operations within a preselected bandwidth of the resonant frequency.
In another embodiment an apparatus includes an ultrasonic transmitter, an ultrasonic transducer coupled to the ultrasonic transmitter, and an ultrasonic receiver coupled to the ultrasonic transducer. The ultrasonic transmitter includes a tunable oscillator which generates an oscillator signal at a selected driving frequency, and a signal generator coupled to the tunable oscillator which generates a driving signal from the oscillator signal by pulsing the oscillator signal between selected start and stop times. The ultrasonic transducer is driven by the driving signal and produces a resonant signal at a resonant frequency when the ultrasonic transmitter stops the driving signal. The ultrasonic receiver includes a signal processor that receives the resonant signal from the ultrasonic transducer and produces a frequency information signal, and a frequency processor coupled to the signal processor that determines the resonant frequency from the frequency information signal and indicates the resonant frequency to the ultrasonic transmitter. The ultrasonic transmitter adjusts the tunable oscillator to maintain the selected driving frequency within a preselected bandwidth of the resonant frequency.